FLAME TEST
Atomic Emission and Electron Energy Levels
Introduction:
Just as a fingerprint is unique to each person, the color of light emitted by an element heated in a flame is also unique to each element. In this experiment, the characteristic color of light emitted by barium, calcium, copper, lithium, potassium, sodium and strontium ions will be observed. Concepts:
Atomic Emission Wavelength and Energy of Light Excited vs. Ground states Flame tests Background:
When a substance is heated in a flame, the atoms absorb energy from the flame. This absorbed energy allows the electrons to be promoted to excited energy levels. From these excited energy levels, there is a natural tendency for the electrons to make a transition or drop back down to the ground state. When an electron makes a transition from a higher energy level to a lower energy level, a particle of light called a photon is emitted (see Figure 1). Both the absorption and emission of energy are quantized – only certain energy levels are allowed. An electron may drop all the way back down to the ground state in a single step, emitting a photon in the process. Alternatively, an electron may drop back down to the ground state in a series of smaller steps, emitting a photon with each step. In either case, the energy of each emitted photon is equal to the difference in energy between the excited state and the state to which the electron relaxes. The energy of the emitted photon determines the color of light observed in the flame. The flame color may be described in terms of its wavelength, and Equation 1 may be used to calculate the energy of the emitted photon. Equation 1: E 
hc

DE is the difference in energy between the two energy level in joules (J), h is the Planck’s constant ( h = 6.626 x 10‐34 J‐sec) c is the speed of light (c = 3 x 108 m/sec), and l (lambda) is the wavelength of light in meters. The wavelengths of visible light are given in units of nanometers (1 m= 1 x 109 nm). See Table 1 on the following page. The color of light observed when a substance is heated in a flame varies from one substance to another. Because each element has a different spacing of electron energy levels, the possible electron transitions for a given substance are unique. Therefore, the difference in energy between energy levels, the exact energy of the emitted photon, and the corresponding wavelength and color are unique to each substance. As a result, the colors observed when a substance is heated in a flame may be used as a means of identification. The Visible Portion of the Electromagnetic Spectrum
Visible light is a form of electromagnetic radiation. Other familiar forms of electromagnetic radiation include g‐rays, X‐
rays, ultraviolet (UV) radiation, infrared (IR) radiation, microwave radiation, and radio waves. Together, all forms of electromagnetic radiation make up the electromagnetic spectrum. The visible portion of the electromagnetic spectrum is the only portion that can be detected by the human eye‐all other forms of electromagnetic radiation are invisible. TM
Adapted from Flinn ChemTopic Labs & Chemistry Lab Manual by Addison‐Wesley. POshikiri Violet Blue Green Yellow Orange Red 400 500 600 700
Figure 2. The visible spectrum The visible spectrum spans the wavelength region from about 400 to 700 nm (Figure 2). Light of 400 nm is seen as violet and light of 700 nm is seen as red. According to Equation 1, wavelength is inversely proportional to energy. Therefore, violet light is higher energy light than red light. As the color of light changes, so does the amount of energy it possesses. Table 1 lists the wavelengths associated with each of the colors in the visible spectrum. The representative wavelengths may be used as a benchmark for each color. For example, instead of referring to green as light in the wavelength range 500‐560 nm, we may approximate the wavelength of a green light as 520 nm. An infinite number of shades of each color may be observed. Representative Wavelength, nm 410 470 490 520 565 580 600 650 Table 1 Wavelength Region, nm 400‐425 425‐480 480‐500 500‐560 560‐580 580‐585 585‐650 650‐700 Color Violet Blue Blue‐green Green Yellow‐green Yellow Orange Red MATERIALS & EQUIPMENT
Q‐Tips (pre‐soaked in water), 2 beakers, 24-well reaction plate, blue glass square
sodium chloride, copper (II) chloride, potassium chloride, calcium chloride, strontium chloride, lithium chloride, barium chloride, “unknown” solid, water PROCEDURE:
1.
Collect small sample amounts of each “known” chemical in a 24-well reaction plate. Keep track of which solid is in
which well. Some solids may look the same in color and texture.
2. Half-fill a beaker with distilled water. Collect enough “soaked” Q-tips and place them in one of the beakers.
3. Light a Bunsen burner so that it has the correct “blue” colored flame.
4. Dip a soaked Q-Tip into one of the wells holding a chemical. Hold the chemical end of Q-Tip on the extreme edge of
the flame of the Bunsen burner. Move it back and forth through the flame.
5. Record the flame color that you observe in the results table.
6. Extinguish the Q-Tip in the other beaker with water.
7. Repeat steps 1 through 6 for the remaining “known” chemical solids and an “unknown” chemical solid provided by
your instructor.
Tips:
* Be very careful not to cross contaminate between chemicals. If you do, you will not be able to distinguish clearly
between the colors produced. Do not place too much solid chemical on the Q-Tip because it will drop some into
the barrel of the Bunsen burner and you won’t be able to distinguish the correct color for a particular metal ion.
* View the potassium flame test through the blue glass square.
* You must observe the colors BEFORE the Q-Tip starts to burn. When it starts to burn it will produce an orange
flame that will obscure the color imparted to the flame by the metal ions.
TM
Adapted from Flinn ChemTopic Labs & Chemistry Lab Manual by Addison‐Wesley. POshikiri DATA TABLE:
METAL ION
COLOR OF FLAME (BE SPECIFIC)
Barium
Calcium
Copper
Lithium
Potassium
Sodium
Strontium
Unknown # ___
Unknown # ___
RESULTS TABLE:
METAL/FLAME COLOR
8 (nm)
8 (m)
ΔE ( J )
POST LAB QUESTIONS:
1. When an atom absorbs energy, the electrons move from their _______________ state to
a(n) ________________ state. When an atom emits energy, the electrons
move from
a(n) ______________ state to their ______________ state and give off ______________.
2. Is a flame test qualitative or quantitative test for the identity of an unknown? Explain.
TM
Adapted from Flinn ChemTopic Labs & Chemistry Lab Manual by Addison‐Wesley. POshikiri 3.
Use Table 1 in the Background section to record the approximate wavelength of light emitted for each
known metal ion in the results table.
4.
Convert each wavelength in the Results Table from nanometers to meters. Show one sample
calculation in the space below and record all values in the Results Table.
5.
The characteristic color of the sodium flame is due to two closely-spaced energy transitions. Use
Equation 1 from the Background section to calculate the average energy (ΔE) corresponding to the
observed flame color for each metal. Show one sample calculation in the space below and record all
values in joules in the Results Table.
6.
Which metal ion has the most energy? Which has the least energy?
7.
Which metal ion emits the shortest wavelength? Which has the longest wavelength?
8.
List the metal ions in order from the highest to lowest frequency.
9.
Identify the unknown compound.
10. The alkali metals cesium (Cs) and rubidium (Rb) were discovered based on their characteristic flame
colors. Cesium is named after the sky and rubidium after the gem color. What colors of light do you
think these metals give off when heated in a flame?
TM
Adapted from Flinn ChemTopic Labs & Chemistry Lab Manual by Addison‐Wesley. POshikiri